This invention relates to the structure of a data recording sheet in which data signals are recorded in a macromolecular material sheet as the distribution of polarization different locally in the direction of the surface of the sheet. The data signals thus recorded are reproduced in a pyroelectric or piezo-electric mode.
It is known in the prior art that some macromolecular material sheets are polarized by temperature and electric field to provide pyroelectricity and piezo-electricity. A method of recording and reproducing data has been proposed in the U.S. Pat. No. 3,772,518 in which, utilizing the fact that the polarization is caused only at the portions of such a sheet on which both temperature and electric field act and the polarization at the portions is eliminated or inverted by applying the opposite electric field together with temperature thereto, data signals are stored as polarization distribution in the macromolecular material sheet. The data signals thus stored are read out when required.
The fact that the polarization signal is both pyroelectric and piezo-electric, is advantageous since the signal stored can be reproduced not only by a pyroelectric method but also by a piezo-electric method. However, the pyroelectric method of reproducing signals is disadvantageous in that the piezo-electric action caused by variations of tension applied to the sheet may cause noise.
Examples of the macromolecular material with which data can be recorded by polarization (hereinafter referred to as "a polarizable macromolecular material" when applicable), are as described in the specification of U.S. Pat. No. 3,772,518. When a sheet of polarizable polymer such as polyvinylidene fluoride polymer, vinyl fluoride polymer, vinylidene fluoride group copolymer or the like is subjected to polarization, the polarity of the piezo-electricity which is generated by applying tension to the sheet is opposite to the polarity of the pyroelectricity which is caused by heating the sheet. Therefore, when the sheet is heated and strained simultaneously, the pyroelectricity and the piezo-electricity may be completely cancelled by each other. Furthermore, the piezo-electricity caused by releasing the tension and having the same sign as the pyroelectricity caused by heating becomes a noise signal.
Another example of the macromolecular material is a mixture of thermo-plastic resin and powder of ferrodielectric non-organic compound such as lead titanate, barium titanate, lead titanium zirconate, or the like. In general, when a non-organic ferrodielectric substance is polarized, the polarity of the piezo-electricity which is caused by straining the substance is the same as that of the pyroelectricity which is caused by heating the substance. Therefore, with a macromolecular material containing a large quantity of ferrodielectric powder, the piezo-electricity is often equal in polarity to the pyroelectricity. If, in this connection, reduction of the tension and heating are effected simultaneously, the piezo-electricity and the pyroelectricity are cancelled by each other.
A system in which recording and reproducing are carried out while a tape-shaped recording material wound on one reel is rewound on another reel, is extensively employed for recording and reproducing data in a tape-shaped recording material. However, this system is disadvantageous in that when the tape is run while being rewound, a tension is necessarily applied to the tape, and the tension varies greatly when the tape begins to run, or is stopped, or when the speed of running the tape is changed.
In addition, the tension is varied also when the reels are not smoothly operated. In this case, the variation of the tension is a periodically repeated vibration. If the piezo-electricity caused by this tension variation has the same polarity as the pyroelectricity caused in the data reproduction, then a noise signal is caused. If the piezo-electricity is opposite in polarity to the pyroelectricity, then the data is cancelled. At any rate, error signals are created.
In the case where the data is reproduced in a piezo-electric mode, the application of pressure must be effectuated perpendicular to the surface of the sheet to detect the difference in polarization between parts of the sheet. Also, in this case, the variation of tension applied to the sheet causes piezo-electric noise signals.